1. Field of the Invention
The present invention relates to a powder based on difunctionally regulated polyamide or copolyamide, to a process for the use of this powder, and also to moldings of this powder.
2. Discussion of the Background
A task often encountered in very recent times is rapid provision of prototypes. Particularly suitable processes are those whose operation is based on pulverulent thermoplastic materials, in which the desired structures are produced layer-by-layer via selective melting and fixing. No support structures are needed here for overhanging and undercut sections, because the powder bed surrounding the melted regions provides sufficient support. There is no need for the subsequent step of removing supports. The processes are also suitable for small production runs.
Laser sintering is a process particularly well suited for rapid prototyping or rapid manufacturing. In this process, polymer powders in a chamber are selectively, briefly irradiated with a laser beam, resulting in melting of the particles of powder on which the laser beam falls. The molten particles fuse and solidify again after cooling to give a solid mass. Complex three-dimensional bodies can be produced simply and relatively rapidly by this process, by repeatedly applying fresh layers and irradiating these.
However, alongside laser sintering there are many other suitable processes. The selectivity of the layer-by-layer processes here can be achieved by way of application of susceptors, of absorbers, of inhibitors, or of masks, or by way of focused introduction of energy, for example via a laser beam or via a glass fiber cable.
A process of laser sintering (rapid prototyping) to realize moldings made from pulverulent polymers is described in detail in the patent specifications U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers is claimed for this application, e.g. polyacetate, polypropylene, polyethylene, ionomers, and nylon-11.
The laser sintering process produces a body in the shape of a block which is composed firstly of the desired components and secondly, usually predominantly, of non-irradiated powder, known as recycling powder, which remains with the components in this block until the molding is revealed, or its covering is removed. This powder supports the components, and overhangs and undercuts can therefore be produced by the laser sintering process without supports. Depending on the nature of the powder used, the non-irradiated powder can be used in a further construction process (recycling) after sieving and addition of virgin powder.
Nylon-12 powder has proven particularly successful in industry for laser sintering to produce engineering components. The parts manufactured from PA 12 powder meet the high requirements demanded with regard to mechanical loading, and therefore have properties particularly close to those of the mass-production parts subsequently produced by extrusion or injection molding.
It is preferable here to use a nylon-12 powder whose melting point is from 185 to 189° C., whose enthalpy of fusion is 112±17 kJ/mol, and whose freezing point is from 138 to 143° C., as described in EP 0 911 142. Use is preferably made here of powders whose median grain size is from 50 to 150 μm, these being obtained as in DE 197 08 946 or else as in DE 44 21 454.
Other processes with good suitability are the SIB process as described in WO 01/38061, or a process as described in EP 1 015 214. Both processes operate with infrared heating to melt the powder. The selectivity of melting is achieved in the first process via application of an inhibitor, and in the second process via a mask. Another process is described in DE 103 11 438. In this, the energy needed for melting is introduced via a microwave generator, and the selectivity is achieved via application of a susceptor. Other suitable processes are those which operate with an absorber, which is either present in the powder or is applied by ink-jet processes, as described in DE 102004012682.8, DE 102004012683.6, and DE 102004020452.7.
A disadvantage of the related art is that the non-irradiated parts of used polyamide powder had a tendency toward post-condensation under the conditions prevailing in the construction chamber of the laser sintering machine (high temperatures, very low moisture level).
As some studies have revealed, the reclaimed polyamide powders and copolyamide powders have markedly increased solution viscosity according to ISO 307, and have only limited capability for use in the next construction process.
In order to achieve consistently good results during processing in a rapid prototyping or rapid manufacturing plant, the related art always mixes the reclaimed powder with considerable amounts of virgin powder. The amounts required of virgin powder are considerably higher than the amounts consumed for the components. The result is an excess of recycling powder which cannot be reused and has to be discarded. Specifically in the case of filigree components, considerable amounts of recycling powder arise in this way, and cannot then be used in further construction processes.
In DE 103 20 590.4, a powder with improved recyclability is described, based on the use of a mono- or dicarboxylic-acid-regulated polyamide. A disadvantage of this related art is impairment of the tensile strain at break of the parts produced therewith. Even in the case of virgin powder, this falls to a value below 10%. This restricts use as a functional prototype or indeed use in rapid manufacturing.
DE 103 305 91.2 achieves an improvement in recyclability via addition of certain additives, such as metal soaps. A disadvantage here is that components thus produced release their additives to some extent in contact with solvents, their use being subject to restriction in certain applications, specifically in contact with food or with the skin.
A uniformly diamine-regulated powder also leads to unsatisfactory results. Although here the solution viscosity remains approximately stable on heating, formation of insoluble or gelled fractions is repeatedly observed on heat-aging, as a consequence of unknown side reactions.